package fr.neyb.framework.observer.impl;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

import fr.neyb.framework.observer.Observable;
import fr.neyb.framework.observer.Observer;

public class AbstractObservable implements Observable {
	private boolean changed = false;
	private List<Observer> obs = Collections
			.synchronizedList(new ArrayList<Observer>());

	@Override
	public synchronized void addObserver(Observer o) {
		if (o == null)
			throw new NullPointerException();
		if (!obs.contains(o)) {
			obs.add(o);
		}
	}

	/**
	 * Deletes an observer from the set of observers of this object. Passing
	 * <CODE>null</CODE> to this method will have no effect.
	 * 
	 * @param o
	 *            the observer to be deleted.
	 */
	@Override
	public synchronized void deleteObserver(Observer o) {
		obs.remove(o);
	}

	/**
	 * If this object has changed, as indicated by the <code>hasChanged</code>
	 * method, then notify all of its observers and then call the
	 * <code>clearChanged</code> method to indicate that this object has no
	 * longer changed.
	 * <p>
	 * Each observer has its <code>update</code> method called with two
	 * arguments: this observable object and <code>null</code>. In other words,
	 * this method is equivalent to: <blockquote><tt>
	 * notifyObservers(null)</tt></blockquote>
	 * 
	 * @see java.util.Observable#clearChanged()
	 * @see java.util.Observable#hasChanged()
	 * @see java.util.Observer#update(java.util.Observable, java.lang.Object)
	 */
	@Override
	public void notifyObservers() {
		notifyObservers(null);
	}

	/**
	 * If this object has changed, as indicated by the <code>hasChanged</code>
	 * method, then notify all of its observers and then call the
	 * <code>clearChanged</code> method to indicate that this object has no
	 * longer changed.
	 * <p>
	 * Each observer has its <code>update</code> method called with two
	 * arguments: this observable object and the <code>arg</code> argument.
	 * 
	 * @param arg
	 *            any object.
	 * @see java.util.Observable#clearChanged()
	 * @see java.util.Observable#hasChanged()
	 * @see java.util.Observer#update(java.util.Observable, java.lang.Object)
	 */
	@Override
	public void notifyObservers(Object arg) {
		/*
		 * a temporary array buffer, used as a snapshot of the state of current
		 * Observers.
		 */
		Observer[] arrLocal = new Observer[obs.size()];

		synchronized (this) {
			/*
			 * We don't want the Observer doing callbacks into arbitrary code
			 * while holding its own Monitor. The code where we extract each
			 * Observable from the Vector and store the state of the Observer
			 * needs synchronization, but notifying observers does not (should
			 * not). The worst result of any potential race-condition here is
			 * that: 1) a newly-added Observer will miss a notification in
			 * progress 2) a recently unregistered Observer will be wrongly
			 * notified when it doesn't care
			 */
			if (!changed)
				return;
			obs.toArray(arrLocal);
			clearChanged();
		}

		for (int i = arrLocal.length - 1; i >= 0; i--)
			(arrLocal[i]).update(this, arg);
	}

	/**
	 * Clears the observer list so that this object no longer has any observers.
	 */
	@Override
	public synchronized void deleteObservers() {
		obs.clear();
	}

	/**
	 * Marks this <tt>Observable</tt> object as having been changed; the
	 * <tt>hasChanged</tt> method will now return <tt>true</tt>.
	 */
	protected synchronized void setChanged() {
		changed = true;
	}

	/**
	 * Indicates that this object has no longer changed, or that it has already
	 * notified all of its observers of its most recent change, so that the
	 * <tt>hasChanged</tt> method will now return <tt>false</tt>. This method is
	 * called automatically by the <code>notifyObservers</code> methods.
	 * 
	 * @see java.util.Observable#notifyObservers()
	 * @see java.util.Observable#notifyObservers(java.lang.Object)
	 */
	protected synchronized void clearChanged() {
		changed = false;
	}

	/**
	 * Tests if this object has changed.
	 * 
	 * @return <code>true</code> if and only if the <code>setChanged</code>
	 *         method has been called more recently than the
	 *         <code>clearChanged</code> method on this object;
	 *         <code>false</code> otherwise.
	 * @see java.util.Observable#clearChanged()
	 * @see java.util.Observable#setChanged()
	 */
	@Override
	public synchronized boolean hasChanged() {
		return changed;
	}

	/**
	 * Returns the number of observers of this <tt>Observable</tt> object.
	 * 
	 * @return the number of observers of this object.
	 */
	@Override
	public synchronized int countObservers() {
		return obs.size();
	}
}
